Efficient degradation of lindane by visible and simulated solar light-assisted S-TiO₂/peroxymonosulfate process: Kinetics and mechanistic investigations

Organochlorine pesticides (OCPs) are toxic and the most potent endocrine disrupting chemicals in the environment. Most OCPs are resistant towards oxidation by [rad]OH due to presence of electron-withdrawing chlorine group in their molecular structures. Here, we investigated a visible and simulated solar light-assisted sulfur doped TiO₂ (S-TiO₂)/peroxymonosulfate (HSO₅⁻) process to eliminate a selected OCP, lindane. Initially, visible and simulated solar light-assisted S-TiO₂ photocatalysis resulted in 31.0 and 63.4% removal of lindane (C₀ = 1.0 μM), respectively in 6 h. The photocatalytic activity of S-TiO₂ was dramatically increased in the presence of 0.2 mM HSO₅⁻, leading to 68.2 and 99.9% lindane removal under visible and simulated solar light illumination, respectively in 6 h. The observed pseudo first-order rate constant for simulated solar light-assisted S-TiO₂/HSO₅⁻ decreased with increasing initial concentration of lindane, corresponding to 8.98 × 10⁻¹, 6.58 × 10⁻¹ and 3.84 × 10⁻¹ h⁻¹ at [lindane]₀ of 0.5, 1.0 and 2.0 μM, respectively. The degradation kinetics were significantly affected by solution pH, leading to 88.2, 99.9 and 71.4% removal of lindane in 6 h at pH 4.0, 5.8 and 8.0, respectively. S-TiO₂ film exhibited a high mechanical strength with only 3.3% loss of efficiency after four repeated cycles. Based on the detected reaction intermediates, a possible reaction mechanism was proposed, suggesting dechlorination, dehydrogenation, and hydroxylation via [rad]OH, SO₄[rad]⁻ and O₂[rad]⁻ attack. The results suggest that visible and simulated solar light-assisted S-TiO₂/HSO₅⁻ is a promising alternative for treatment of water contaminated with most OCPs.